Voltage regulators that provide clean output signals are essential to the performance of analog integrated circuits. This is especially true when analog integrated circuits are used in portable electronic devices such as digital cameras, cellular phones, and laptop computers that require low quiescent currents and a low supply voltage from 1.9 volts to 3.3 volts. Transient responses of these analog integrated circuits can cause irreversible failure and often shorten the lifetime of these portable devices. Low drop-out voltage regulator circuits are widely used because they provide a stable, low noise and well specified DC output voltage for integrated circuits. However, low drop-out voltage regulator circuits are vulnerable to transient overshoot and undershoot errors that are caused by the switching on and off of a downstream load device.
With reference to the schematic diagram of FIG. 1A, a prior art low drop-out voltage regulator circuit 100 connected to a downstream load device represented by a load current 112 is shown that includes an error amplifier 101, a pass device 102, and a reference network including a first resistor (R1) 103 and a second resistor (R2) 104. Low drop-out voltage regulator circuit 100 produces an output voltage (VOUT) independent of the input voltage (VIN) and proportional to a reference voltage (VREF). FIG. 1B shows a waveform 119 representing load current 112 and a waveform 120 representing the output signal (VOUT). When load current 112 is turned on, the current increases from 0 mA to 500 mA which is represented by a rising edge 119U of waveform 119. In response, the voltage on capacitor 105 decreases but low drop-out voltage regulator circuit 100 does not react fast enough to compensate for the sudden drop in the output voltage (VOUT). As a result, an undershoot error 121 exists in the transient response of the output signal (VOUT). The magnitude of undershoot error 121 is ΔV−=IL·Δt/C (Equation 1), where C is the capacitance of capacitor 105.
Continuing with FIG. 1B, when load current 112 is turned off, the current decreases from 500 mA to 0 mA which is represented by a falling edge 119D of waveform 119. In response, the voltage of capacitor 105 increases but low drop-out voltage regulator circuit 100 does not react fast enough to stop compensating for the output voltage (VOUT) which is represented by a waveform 120. This results in an overshoot error 122 in output waveform 120. The magnitude of overshoot error 122 is ΔV+=IL·Δt/C (Equation 2). Typically, in order to solve undershoot error 121 and overshoot error 122 in low drop-out voltage regulator circuit 100, a large capacitor 105 between 10 μF and 100 μF is connected to the output terminal 109 and an electrical ground 111. Large capacitance C in the denominator of Equation 1 and Equation 2 reduces the magnitude of ΔV− and ΔV+. However, large capacitor 105 requires a significant amount of board area as well as increases in manufacturing costs. In addition, large capacitor 105 tends to slow the response time of low drop-out voltage regulator circuit 100. On the other hand, reducing the value of capacitor 105 speeds up the reaction time but may cause instability and an increase the overshoot (equation 2) in low drop-out voltage regulator circuit 100. Therefore, changing the capacitance (C) of capacitor 105 is not feasible to handle overshoot and undershoot problems. Another method is to reduce the reaction time (Δt) in equations 1 and 2 by using very fast error amplifier 101. However, fast error amplifiers require expensive process technology and complex circuit design. Thus, additional circuitry is needed to solve the transient overshoot and undershoot errors in low drop-out voltage regulator 100.
Many prior arts have attempted to provide additional circuitry to solve the overshoot and undershoot errors problems in low drop-out voltage regulator 100. In one prior art, an output stage compensation circuit electrically coupled between error amplifier 101 and pass device 102 is disclosed. Inside the prior art's output stage compensation circuit, one or more segmented sense devices are configured to provide pole-zero compensation to low drop-out voltage regulator circuit 100 based on output current. Each of the segmented sense devices is configured to compensate a suitable range of output current and to multiply the effect of associated compensation capacitors. As a result, the output stage of compensation circuit of the prior art provides a stable output voltage (VOUT) which is not dependent upon the output current and the capacitance requirements. However, the disclosed output stage compensation circuit does not address overshoot error and undershoot error problems. Furthermore, the prior art circuits do not provide a solution to low quiescent current requirements and economy in silicon area.
Accordingly, there are unmet needs for a method and an overshoot and undershoot correction circuit that enable low drop-out voltage regulator circuits to achieve fast reaction time so as to solve for the overshoot error and undershoot error problems. In addition, there is an unmet need for an overshoot and undershoot correction circuit that does not consume a large amount of quiescent currents and occupy large circuit board area. The present invention meets the above needs.